1. Field of the Invention
This invention relates to a bucket type rotary excavator for use in foundation work (for cast-in-place piles) or the like for civil engineering and construction works.
2. Description of the Prior Art
In case of execution of a cast-in-place pile in the ground a, it is advantageous to expand the lower end of a column body b of the pile as shown in FIG. 1, because the expanded end increases its bearing power.
The reverse circulation method for mechanical excavation has been used for the execution of such a pile having an expanded lower end. In excavating the ground a by rotary bits (not shown) according to the reverse circulation method, a stand pipe d is driven in the proximity of the surface of the ground and a slurry of bentonite or slime water is filled in the pile hole to prevent inner walls f of the hole from falling down as shown in FIG. 2. On the other hand, the rotary bits are rotated and forced downward to excavate the ground. The excavated earth and sand are exhausted out of the pile hole with the aid of the reverse circulation of the slurry or water.
In this reverse circulation method, however, the expanded bottom g of the pile hole is unavoidably inverted conical as shown at h in FIGS. 1 and 2, so that it contains precipitated slime i at its bottom. After placing of concrete in the pile hole to form a foundation pile, the stayed slime tends to reduce the bearing power of the foundation pile at the bottom to decrease its reliability.
In order to solve these problem, the applicant had proposed a divided bucket type rotary excavator comprising cutter units constructed by dividing a bottomed cylindrical bucket into plural portions which are arranged about a rotatable shaft and adapted to move radially toward and away from the rotatable shaft (Japanese Patent Application Publication No. 21,079/83 patented as Japanese Pat. No. 1,190,290). The invention of the present application relates to an improvement of the excavator of the Japanese Patent.